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A low phase-noise X-band monolithic-microwave integrated-circuit voltage-controlled oscillator (VCO) based on a novel high-linearity and low-noise composite-channel A10.3 Ga0.7N/A10.05 Ga0.95 N/GaN high electron mobility transistor (HEMT) is presented. The HEMT has a 1 μm × 100 μm gate. A planar inter-digitated metal-semiconductor-metal varactor is used to tune the VCO's frequency. The polyimide dielectric layer is inserted between a metal and GaN buffer to improve the Q factor of spiral inductors. The VCO exhibits a frequency tuning range from 9.11 to 9.55 GHz with the varactor's voltage from 4 to 6 V, an average output power of 3.3 dBm, and an average efficiency of 7% at a gate bias of -3 V and a drain bias of 5 V. The measured phase noise is -82 dBc/Hz and -110 dBc/Hz at offsets of 100 kHz and 1 MHz at a varactor's voltage (Vtune) = 5 V. The phase noise is the lowest reported thus far in VCOs made of GaN-based HEMTs. In addition, the VCO also exhibits the minimum second harmonic suppression of 47 dBc. The chip size is 1.2 × 1.05 mm2. [ABSTRACT FROM AUTHOR]